AP Physics 1 & 2 Light As a Wave

The light that you see is simply an energy packet emitted from a source, which is also known as a wave. Since light is a type of wave, and since we know how to mess with waves, it stands to reason that we can modify light with interferers. These interferers are called polarizers, which basically block some parts of light and prevent it from passing through. Anything that is built to let some but not all light through is a type of polarizer by nature. Alternatively, you can alter light without polarizing it by using things called mirrors.

Light reflecting of non-metallic surfaces is partially polarized parallel to the reflecting surface.

Light As a Wave

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

Transcription: Light As a Wave

Todays lesson is going to be on light as a wave, so we are going to start transitioning from waves into a specific type of wave -- electromagnetic waves or light and optics.0003

Our objectives are going to be to recognize that light is an electromagnetic wave and shares characteristics with the entire electromagnetic spectrum and explain the concept of polarization and how polarization can be used in applications, such as video displays and low reflection sunglasses.0014

EM waves do not require a medium in which to propagate; they can move through a vacuum.0035

Light is an electromagnetic wave, which is visible to the human eye, but there are lots of other types of electromagnetic waves that we cannot see as well.0041

The speed of all electromagnetic waves in a vacuum is approximately 3 × 108 m/s, probably the most important constant in this entire course.0049

Now electromagnetic waves are transverse waves due to the modulation -- the variation and the electromagnetic field's perpendicular to the wave velocity.0060

I have a diagram down here, so if we have the wave traveling with a velocity to the right, the electric field of the EM wave is going to vary in a plane perpendicular to that velocity and the magnetic field is going to vibrate or oscillate.0069

It is going to modulate in a direction perpendicular to both the electric field and to the wave velocity.0084

That is what makes it a transverse wave, even though there is no medium required.0091

Let us talk about characteristics of electromagnetic waves and the electromagnetic spectrum.0097

The product of an electromagnetic waves frequency in wave length must be constant in a vacuum.0103

If V = F(λ) and in a vacuum, V = C or 3 × 108, frequency times wavelength must be constant.0108

Higher frequencies then, must have shorter wavelengths and lower frequencies must have longer or bigger wavelengths.0117

The relationship between frequency and wavelength for various types of electromagnetic waves is shown in the electromagnetic spectrum.0124

I have a diagram here, although you can find tons of different diagrams of the electromagnetic spectrum.0130

Here is just one version of it -- as we look, light is just this little bit here in the middle.0135

Now it has expanded out so you can see the different colors here, but all of these other waves are electromagnetic waves.0142

Now we start over here on the left and these are the high energy waves; they have the highest energy and they also have the highest frequency.0149

The energy of an electromagnetic wave is related to its frequency -- more frequency, more energy -- so if things like gamma rays have a ton of energy, they have a very high frequency and therefore a very short wavelength.0158

So we move to the right along the scales and we hit x-rays -- not quite as much energy as gamma rays, but still pretty energetic -- a little bit lower frequency, a little bit longer wavelength, but still pretty short wavelength.0171

We get into the visible spectrum, which starts ultraviolent, near violet and has more energy and as we go the right we get to the lower and lower frequencies -- the longer wavelengths and less energy.0188

Infrared, microwaves, radio waves, of which TV, FM, and AM are different types and long radio waves and so on.0201

Polarized light, however, consists of light where we have the oscillations -- the modulations of the electric and magnetic fields in a single direction only.0246

Here, think of this as the electric field vibrating in one direction and we are simplifying this a little bit just until we get the concept across; there is certainly some more complexity to it as we get into more depth.0254

What does that allow us to do? We could talk about things like polarizers.0267

Polarizers are materials which act like filters to allow only specific polarizations of light to pass.0273

If you have all these different polarizations of light at different angles coming toward you -- if you had a picket fence where all of the fence rows are lined up, only certain polarizations that line up with the polarization of your picket fence are going to make it through.0280

You are blocking the light that does not fit the polarization of what we will call your polarizer, your picket fence.0296

So you have taken un-polarized light and only let one polarization of light get through, so you have now made polarized light.0301

Polarizers are typically sheets of material in which you have a bunch of long molecules lined up, kind of like a picket fence.0310

Of course they have to be a lot smaller because the wavelength of light is so much smaller.0316

Let us take a look here. A microwave and an x-ray are traveling in a vacuum.0633

Compared to the wavelength and period of the microwave, the x-ray has a wavelength that is...?0637

Well, let us see. If we go back to our electromagnetic spectrum or even think about energy, the x-ray had a higher frequency; it had more energy.0642

So the frequency of the x-ray was higher and we know that if its frequency is higher, then its period is 1 over frequency, so that has to be shorter.0651

So frequency is higher, the period is shorter, and if V = F(λ) -- V is constant and if (F) is higher, then wavelength must be smaller, so we must have a wavelength that is shorter and a period that is shorter. Answer D.0663

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